Due to the gradually declining production of crude oil in the U.S.A., a number of enhanced oil recovery (EOR) projects have been undertaken to stimulate production from known oil fields that are beyond their economic recovery limits. One of these EOR methods inject their surfactant into the reservoir which lowers the oil-brine interfacial tension and allows more oil to be produced. Such processes are commonly known as surfactant, surfactant-polymer, micellar, micellar-polymer floods.
Although the surfactant flooding systems have proved successful in recovering additional oil, they have also introduced problems which have heretofore not been encountered. Foremost among these is that the produced oil is an emulsion caused by the surfactants, cosurfactants and polymers used in surfactant floods utilized in secondary and tertiary recovery processes for oil.
Fluids produced by the flooding of an oil reservoir with a surfactant flood generally consist of three phases each of which is an emulsion comprised of surfactant, brine and oil. The amounts of the three phases, relative to one another, change throughout the course of the flood. For example, with respect to a micellar-polymer flood, at the height of oil production, the relative amounts of the three phases might characteristically be:
Upper phase 13.+-.5% PA1 Middle phase 2.+-.2% PA1 Lower phase 87.+-.7% PA1 A.sub.1 and A.sub.2 may be the same or different alkyls; PA1 n is an integer having a value of at least one which denotes the number of --OA.sub.1 OA.sub.2 N groups attached to R.sub.1 ;
The upper-phase emulsion (UPE) consists largely of oil and is stabilized by oil-soluble surfactants. It can contain as much as 12% water and surfactants; however, this total is generally less than about 5%.
The middle-phase emulsion (MPE) is a mixture of about equal quantities of oil and brine. It is stabilized by divalent ion sulfonates and the polymer, e.g., polyacrylamide, used for mobility control of the flood.
The lower-phase emulsion (LPE) is largely brine. It is stabilized by water-soluble surfactants and also contains salts. Additionally, it may contain as much as 5% oil in addition to a water-soluble polymer. The UPE is a water-in-oil microemulsion; the LPE is an oil-in-water microemulsion; and the MPE is a macroemulsion.
The three emulsion phases can be separated by allowing the produced fluids to stand quiescently. If acceleration rates are desired, demulsifiers may be added or the fluids can be centrifuged. The addition of economically attractive amounts of demulsifiers, i.e., less than about 0.5 percent by weight based on the weight of the oil, do not significantly alter the volumes of the compositions of the three emulsion phases from comparable values obtained by quiescent standing in the absence of demulsifiers.
Therefore, due to the diverse natures of the three emulsion phases, no single demulsifier or combination of the emulsifiers has been discovered which will demulsify all three phases simultaneously or even the upper and middle phases simultaneously. In fact, it is not uncommon to find that emulsifiers behave antagonistically toward toward one another. For example, a particular demulsifier may excel in separating and oil-in-water emulsion, but when added to an oil-in-water emulsion, it may have the opposite effect and aid in stabilizing the emulsion. Moreover, generally attempts to demulsify both the upper phase and middle phases of a surfactant produced crude oil require large amounts of conventional nonionic surfactants which may cause the middle phase to be dissolved in the oil to yield a two phase system. This is of no benefit to subsequent processing of the oil, since the middle phase emulsion and the oil must eventually be separated. In short, none of the prior art demulsifiers, which are usually nonionic demulsifiers, are useful in the simultaneous demulsification of all three phases nor are any of them known for their ability to demulsify the middle phase emulsion.
U.S. Pat. No. 4,089,803 is representative of conventional type demulsifiers. It discloses a demulsifier consisting of a mixture of an oxyalkylated phenol-aldehyde resin, a polyethylene amine and an alkanol amine. The oxyalkylated phenol-aldehyde resin may be a phenol-formaldehyde polymer which has been subsequently reacted with ethylene oxide to achieve the desired degree of ethoxylation. The polyethylene amine may be pentaethylene hexamine, and the alkanol amine may be triethanol amine. Demulsifiers of this type have been marketed under the tradenames Tretolite P-453 and Tretolite RP-578.